1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a gettering technique. Specifically, the present invention relates to a method of manufacturing a semiconductor device from a semiconductor film which has a crystal structure and which is doped with a metal element for accelerating crystallization thereof.
The term semiconductor device in this specification refers to devices in general that utilize semiconductor characteristics to function, and electrooptical devices, semiconductor circuits, and electronic equipment are all deemed as semiconductor devices.
2. Description of the Related Art
Thin film transistors (hereinafter referred to as TFTs) are known as typical semiconductor elements that use semiconductor films having a crystal structure. TFTs are attracting attention as a technique of forming an integrated circuit on a glass or other insulating substrate, and devices utilizing TFTs, such as a liquid crystal display device with a built-in driving circuit, are beginning to appear on the market. In prior art, a semiconductor film with a crystal structure is formed by using heat treatment or laser annealing to crystallize an amorphous semiconductor film that is obtained by deposition through plasma CVD or low pressure CVD. (Laser annealing is the technique of crystallizing a semiconductor film through irradiation of laser light.)
The thus formed semiconductor film with a crystal structure is a mass of crystal grains. Since the crystal grains are randomly oriented and the orientation thereof cannot be controlled, the semiconductor film affects TFT characteristics. Japanese Patent Application Laid-open No. Hei 7-183540 discloses a technique to tackle this problem. The technique involves doping with a metal element that accelerates crystallization of a semiconductor film, such as nickel, to form a semiconductor film having a crystal structure. The technique can cause a large proportion of crystal grains to orient in the same direction, and can lower the heating temperature required for crystallization as well. When this semiconductor film having a crystal structure is used in a TFT, the field effect mobility is improved and the sub-threshold coefficient (S value) is reduced to improve the electric characteristics of the TFT exponentially.
On the other hand, the metal element used in doping for accelerating crystallization remains in the semiconductor film having a crystal structure, or on the surface thereof, causing problems such as fluctuation in characteristic of semiconductor elements obtained. For example, the remaining metal element increases OFF current in the TFTs to cause fluctuation between the semiconductor elements. In short, the metal element for accelerating crystallization becomes an unwanted presence once the semiconductor film having a crystal structure is formed.
Gettering using phosphorus is actively employed as an effective method of removing a metal element that accelerates crystallization from a specific region of a semiconductor film having a crystal structure. For instance, the metal element can readily be removed from a channel formation region by doping a source or drain region of a TFT with phosphorus and subjecting the film to heat treatment at 450 to 700° C.
Phosphorus is injected to the semiconductor film having a crystal structure by ion doping (ion doping is a method of dissociating PH3 or the like by plasma and accelerating the obtained ions in the electric field to inject the ions into a semiconductor, and basically does not include ion mass separation). For gettering, the concentration of phosphorus in the semiconductor film has to be 1×1020/cm3 or higher. Phosphorus doping by ion doping makes the semiconductor film having a crystal structure amorphous, and an increased phosphorus concentration inhibits recrystallization during the subsequent annealing. In addition to this problem, high concentration phosphorus doping prolongs treatment time required for doping and lowers throughput in the doping step.
Furthermore, a source or drain region of a p-channel TFT which is doped with phosphorus needs boron in a concentration 1.5 to 3 times higher than the phosphorus concentration in order to reverse the conductivity type of the region. This makes the recrystallization difficult and raises the resistance of the source or drain region undesirably.